This project examines the effect of continuous positive airways pressure (CPAP) treatment on the sympathetic nervous system (SNS) of sleep apneic patients. In previous studies we have documented that apneics have increased SNS tone, as inferred by urinary catecholamines, beta adrenergic receptors, preejection period, Heather index, cardiac acceleration index. Such individuals also demonstrate alterations in baroreflex functioning (increased pressor sensitivity). Numerous other studies have reported that CPAP treatment lowers the blood pressure of apneic patients. The mechanism and time course of such blood pressure lowering is uncertain. We will enroll 60 patients with sleep apnea and characterize their blood pressure and SNS activity. Patients will then be randomized to receive a one week trial on either CPAP or placebo-CPAP (insufficient CPAP pressure to reduce the respiratory disturbances). Their blood pressure and sympathetic nervous system activity will be studied after 1 day and after 7 days on CPAP (or placebo-CPAP). End points will include: ambulatory blood pressure, baseline blood pressure, blood pressure in response to behavioral stressors, plasma and urinary catecholamines, measures of cardiac SNS tone as inferred from impedance cardiography, adrenergic receptor characteristics, and baroreflex functioning.